How to display data using WP_List_Table ? My code is like below

class myTable extends WP_List_Table
{
    var $example_data = array(
        array('ID' => 1,'booktitle' => 'Quarter Share', 'author' => 'Nathan Lowell',
                'isbn' => '978-0982514542'),
        array('ID' => 2, 'booktitle' => '7th Son: Descent','author' => 'J. C. Hutchins',
                'isbn' => '0312384378'),
        array('ID' => 3, 'booktitle' => 'Shadowmagic', 'author' => 'John Lenahan',
                'isbn' => '978-1905548927'),
        array('ID' => 4, 'booktitle' => 'The Crown Conspiracy', 'author' => 'Michael J. Sullivan',
                'isbn' => '978-0979621130'),
        array('ID' => 5, 'booktitle'     => 'Max Quick: The Pocket and the Pendant', 'author'    => 'Mark Jeffrey',
                'isbn' => '978-0061988929'),
        array('ID' => 6, 'booktitle' => 'Jack Wakes Up: A Novel', 'author' => 'Seth Harwood',
                'isbn' => '978-0307454355')
    );

    function get_columns(){
        $columns = array(
            'booktitle' => 'Title',
            'author'    => 'Author',
            'isbn'      => 'ISBN'
        );
        return $columns;
    }

    function prepare_items() {
        $columns = $this->get_columns();
        $hidden = $this->get_hidden_columns();
        $sortable = $this->get_sortable_columns();
        $this->_column_headers = array($columns, $hidden, $sortable);
        $this->items = $this->example_data;
    }

    function column_default( $item, $column_name ) {
        switch( $column_name ) {
            case 'booktitle':
            case 'author':
            case 'isbn':
                return $item[ $column_name ];
            default:
                return print_r( $item, true ) ;   //Show the whole array for troubleshooting purposes
        }
    }

    public function get_hidden_columns()
    {
        return array();
    }

    public function get_sortable_columns()
    {
        return array('title' => array('title', false));
    }
}

But I can't see anything.